It has previously been proposed to provide magneto-type ignition systems by utilizing a switching semiconductor, such as a transistor which is controlled first to conduction to store magnetic energy in an ignition coil and then to cut off, or blocked state, the abrupt transition of the transistor causing an inductive pulse in the secondary, which causes breakdown of the spark gap of a spark plug, thereby causing an ignition event. The system is so arranged, that at the beginning of positive voltage half wave in the primary circuit, the ignition transistor is first controlled to conductive state to cause primary current to flow. The armature of the magneto generator will, accordingly, store electric energy due to a strong magnetic field occurring in the magneto. In the range of the peak value of the voltage half wave, the switching transistor is controlled to cut off, causing the ignition event.
It is desirable to change the timing of the ignition event with respect to upper dead center (UDC) position of the piston of the internal combustion engine. Timing, and typically spark advance has previously been controlled by the magneto itself. The magneto was constructed to generate first a smaller and thereafter a larger positive voltage half wave in the primary circuit. At low engine speeds of the internal combustion (IC) engine, a threshold switch which forms part of the circuit would be triggered to initiate the ignition event only when the voltage half wave has reached a certain value, which could only occur at the higher or second voltage half wave. Since, with increasing speed, the voltages in the primary circuit also increase, the threshold switch would respond in an upper speed range already when the smaller voltage half wave occurs, which occurs earlier than the larger one. Thus, the ignition event is advanced, that is, ignition is advanced with respect to UDC position of the piston The advance occurs at a predetermined speed, that is, when the smaller half wave reaches the threshold level of the threshold switch. The transition between timing of the ignition event by the main wave and by the auxiliary high speed control wave is sudden and abrupt. This system is described in detail in U.S. Pat. No. 3,963,015, Haubner et al, assigned to the assignee of this application.
The efficiency of IC engines, which operate at wide range of speeds depends to a substantial extent on ignition timing. In intermediate speed ranges, therefore, an intermediate ignition advance is desired and operating with respect to two discrete timing positions is undesirable. The ignition advance-engine characteristic is not matched efficiently to the load requirements and operating conditions of the engine. Generations of an additional auxiliary triggering wave to advance ignition timing causes additional expense due to additional constructional elements of the magneto itself.